1. Field of the Invention
The invention relates to electrical and optical devices that incorporate crystalline group III-nitrides.
2. Discussion of the Related Art
Crystalline group III-nitride semiconductors are used in both electrical devices and optical devices.
With respect to electrical devices, group III-nitrides have been used to make field-emitters. A field-emitter is a conductive structure with a sharp tip. The sharp tip produces a high electric field in response to being charged. The high electric field causes electron emission from the tip. For this reason, an array of field emitters can operate a phosphor image screen.
One prior art method has fabricated arrays of field-emitters from group III-nitrides. Group III-nitrides have chemical and mechanical stability due to the stability of the group III atom-nitrogen bond. Such stability is very desirable in devices that use an array of field-emitters.
The prior art method grows the field emitters from group III-nitrides. The growth method includes epitaxially growing a gallium nitride (GaN) layer on a sapphire substrate, forming a SiO2 mask on the GaN layer, and epitaxially growing pyramidal GaN field-emitters in circular windows of the mask. While the growth method produces field-emitters of uniform size, the field emitters do not have very sharp tips. Sharper tips are desirable to produce higher electron emission rates and lower turn on voltages.
With respect to optical devices, group III-nitrides have high refractive indices. Materials with high refractive indices are desirable in the manufacture of photonic bandgap structures. For a fixed photonic bandgap, such materials enable making a photonic bandgap structure with larger feature dimensions than would be possible if the structure was made from a lower refractive index material.
One method for making a planar photonic bandgap structure involves dry etching a smooth layer of group III-nitride. Unfortunately, the chemical stability of group III-nitrides causes dry etchants to have a low selectivity for the group III-nitride over mask material. For that reason, a dry etch does not produce a deep surface relief in a layer of group III-nitride. Consequently, the thy-etch method only produces thin planar photonic bandgap structures from group III-nitrides.
Unfortunately, light does not efficiently edge couple to thin planar structures. For this reason, it is desirable to have a method capable of fabricating a photonic bandgap structure with a higher surface relief from a group III-nitride.